The restoration of decayed dental structures including caries, decayed dentin or decayed enamel, is often accomplished by the sequential application of a dental adhesive and then a dental material (e.g., a restorative material) to the relevant dental structures. Similar compositions are used in the bonding of orthodontic appliances (generally utilizing an orthodontic adhesive) to a dental structure. Often various pretreatment processes are used to promote the bonding of adhesives to dentin or enamel. Typically, such pretreatment steps include etching with, for example, inorganic or organic acids, followed by priming to improve the bonding between the tooth structure and the overlying adhesive.
A variety of dental and orthodontic adhesives, cements, and restoratives are currently available. Compositions including fluoroaluminosilicate glass fillers (also known as glass ionomer or “GI” compositions) are among the most widely used types of dental materials. These compositions have a broad range of applications such as filling and restoration of carious lesions; cementing of, for example, a crown, an inlay, a bridge, or an orthodontic band; lining of cavity; core construction; and pit and fissure sealing.
There are currently two major classes of glass ionomers. The first class, known as conventional glass ionomers, generally contains as main ingredients a homopolymer or copolymer of an α,β-unsaturated carboxylic acid, a fluoroaluminosilicate (“FAS”) glass, water, and optionally a chelating agent such as tartaric acid. These conventional glass ionomers typically are supplied in powder/liquid formulations that are mixed just before use. The mixture undergoes self-hardening in the dark due to an ionic acid-base reaction between the acidic repeating units of the polycarboxylic acid and cations leached from the basic glass.
The second major class of glass ionomers is known as hybrid glass ionomer or resin-modified glass ionomers (“RMGI”). Like a conventional glass ionomer, an RMGI employs an FAS glass. An RMGI also contains a homopolymer or copolymer of an α,β-unsaturated carboxylic acid, an FAS glass, and water; however, the organic portion of an RMGI is different. In one type of RMGI, the polyacid is modified to replace or end-cap some of the acidic repeating units with pendent curable groups and a photoinitiator is added to provide a second cure mechanism. Acrylate or methacrylate groups are typically employed as the pendant curable group. In another type of RMGI, the composition includes a polycarboxylic acid, an acrylate or methacrylate-functional monomer or polymer, and a photoinitiator. The polyacid may optionally be modified to replace or end-cap some of the acidic repeating units with pendent curable groups. A redox or other chemical cure system may be used instead of or in addition to a photoinitiator system. RMGI compositions are usually formulated as powder/liquid or paste/paste systems, and contain water as mixed and applied. They may partially or fully harden in the dark due to the ionic reaction between the acidic repeating units of the polycarboxylic acid and cations leached from the glass, and commercial RMGI products typically also cure on exposure of the cement to light from a dental curing lamp.
There are many important benefits provided by glass ionomer compositions. For example, fluoride release from glass ionomers tends to be higher than from other classes of dental compositions such as metal oxide cements, compomer cements, or fluoridated composites, and thus glass ionomers are believed to provide enhanced cariostatic protection. Another advantage of glass ionomer materials is the very good clinical adhesion of such cements to tooth structure, thus providing highly retentive restorations. Since conventional glass ionomers do not need an external curing initiation mode, they can generally be placed in bulk as a filling material in deep restorations, without requiring layering.
One of the drawbacks of conventional glass ionomers is that these compositions are somewhat technique sensitive when mixed by hand. They are typically prepared from a powder component and a liquid component, thus requiring weighing and mixing operations prior to application. The accuracy of such operations depends in part on operator skill and competency. When mixed by hand, the powder component and the liquid component are usually mixed on paper with a spatula. The mixing operation must be carried out within a short period of time, and a skilled technique is needed in order for the material to fully exhibit the desired characteristics (i.e., the performance of the cement can depend on the mixture ratio and the manner and thoroughness of mixing). Alternatively, some of these inconveniences and technique sensitivities have been improved by utilization of powder liquid capsule dispensing systems that contain the proper proportion of the powder and liquid components. While capsules provide proper proportions of the powder and liquid components, they still require a capsule activation step to combine the two components followed by mechanical mixing in a dental triturator.
Conventional glass ionomers may also be quite brittle as evidenced by their relatively low flexural strength. Thus restorations made from conventional glass ionomers tend to be more prone to fracture in load bearing indications. In addition, glass ionomers are often characterized by high visual opacity (i.e., cloudiness), especially when they come into contact with water at the initial stage of hardening, resulting in relatively poor aesthetics.
Cured RMGIs typically have increased strength properties (e.g., flexural strength), are less prone to mechanical fracture than conventional glass ionomers, and typically require a primer or conditioner for adequate tooth adhesion.